Final answer:
The Lewis model for covalent bonding accounts for the stability of atom combinations based on electronegativity. If there is a large difference in electronegativity, the bond is ionic, while a small or non-existent difference results in a covalent bond.
Step-by-step explanation:
The Lewis model for covalent bonding accounts for why certain combinations of atoms are stable while others are not based on electronegativity. Electronegativity is the ability of an atom to attract a pair of electrons in a chemical bond. If there is a large difference in electronegativity between two atoms, the bond is characterized as ionic. On the other hand, if the electronegativity difference is small or non-existent, the bond is covalent.